With the advent of the multimedia era, the applications of the display device will be expanded. Various electronic apparatus having a display device has been well known and widely used gradually. People often read the texts or view the pictures through electronic apparatus, and can also watch the videos through electronic apparatus. An electronic apparatus having a display device typically has two screen modes, the portrait mode and the landscape mode. Generally, the electronic apparatus determines the screen mode automatically through the sensing signal of the inertial sensor (for example, G-sensor). The function that the electronic apparatus automatically determines the screen mode is called auto orientation. However, if the screen of the electronic apparatus is parallel to the ground (e.g., the user uses the electronic apparatus when lying down), the G-sensor cannot determine the direction correctly, which causes the electronic apparatus to display the frame in wrong screen mode. For example, the user holds the electronic apparatus in landscape mode, but the electronic apparatus displays the frame in portrait mode; or the user holds the electronic apparatus in portrait mode, but the electronic apparatus displays the frame in landscape mode. Therefore, how to correctly determine the display orientation of the electronic apparatus has become an important topic in the industry.